1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium and, more particularly, to a reversible thermosensitive recording medium comprising a support, a reversible thermosensitive recording layer formed thereon capable of reversibly changing its transparency from a transparent state to a white opaque state, and vice versa, depending upon the temperature thereof, and a magnetic recording layer which is integrally formed in a part of the reversible thermosensitive recording layer. In addition, the present invention relates to a method of producing the above-mentioned reversible thermosensitive recording medium.
2. Discussion of Background
Recently attention has been paid to a reversible thermosensitive recording medium capable of temporarily recording images thereon and erasing the same therefrom when such images become unnecessary. For example, there is conventionally known a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer in which an organic low-molecular-weight material such as a higher fatty acid is dispersed in a matrix resin with a low glass transition temperature, such as a vinyl chloride-vinyl acetate copolymer, as disclosed in Japanese Laid-Open Patent Applications 54-119377 and 55-154198.
Furthermore, between a support and a reversible thermosensitive recording layer of the above-mentioned reversible thermosensitive recording medium, the provision of a colored layer is proposed as disclosed in Japanese Laid-Open Utility Model Application 2-3876; the provision of alight reflection layer, in Japanese Laid-Open Patent Application 64-14079; and the provision of a thin layer with a refractive index different from that of the reversible thermosensitive recording layer in a transparent state, in Japanese Laid-Open Patent Application 2-175280. In addition, a variety of protective layers are proposed for this kind of reversible thermosensitive recording medium. For example, there are proposed a protective layer comprising a silicone rubber or a silicon resin as disclosed in Japanese Laid-Open Patent Application 62-221087; a protective layer comprising a polysiloxane graft polymer as disclosed in Japanese Laid-Open Patent Application 63-317385; and a protective layer comprising an ultraviolet-curing resin or electron radiation curing resin as disclosed in Japanese Laid-Open Patent Application 2-566.
A recording medium comprising a magnetic recording layer and a thermosensitive recording layer is actively developed, and this kind of recording medium has been put to practical use as a passenger ticket and a commutation ticket. In addition, a magnetic recording card provided with a thermosensitive recording layer capable of indicating the used condition of the card is disclosed in Japanese Laid-Open Patent Applications 59-199284 and 60-18388, and such a recording card has also been put to practical use as a prepaid card for paying the charge for the public telephone and the fare for the railway. Further, the holders of the prepaid card or cash card wants to know the balance each time the prepaid card or case card is used. In compliance with such a request, there is proposed a magnetic recording card with a reversible thermosensitive recording layer capable of indicating the information such as a balance of the card, and rewriting the information, as disclosed in Japanese Laid-Open Patent Application 1-181766.
As previously mentioned, a recording medium capable of carrying out both magnetic recording and thermal recording has achieved a rapid development. In the development of this kind of recording medium, a magnetic recording layer and a thermosensitive recording layer are generally provided on the opposite sides of a support of the recording medium. When the magnetic recording layer and the thermosensitive recording layer are provided on the same side of the support, the thermosensitive recording layer is commonly overlaid on the magnetic recording layer in order to facilitate the recognition of information displayed on the thermosensitive recording layer. In this case, however, the output from the magnetic recording layer is weakened and reading errors frequently occur when the thermosensitive recording layer is thick. On the contrary, when the thickness of the thermosensitive recording layer is decreased, the image density of images formed on the thermosensitive recording layer is decreased, so that it is difficult to recognize the images.
When the reversible thermosensitive recording layer and the magnetic recording layer are provided on the same side of the support of a recording medium, the following advantages can be obtained:
(1) An apparatus for recording and reading information can be disposed on one side of the recording medium. PA1 (2) Another magnetic recording layer can be provided on the other side, so that the magnetic recording capacity can be increased. PA1 (3) The side of the recording medium on which no recording layer is provided can be freely designed.
In light of the above-mentioned advantages, various improvements have been introduced in such a recording medium comprising a magnetic recording layer and a reversible thermosensitive recording layer which are provided on the same side of the support. For instance, there is proposed an information recording and display medium which comprises a support, and a magnetic recording layer, a light reflection layer with a thickness of 0.02 to 0.1 .mu.m and a reversible thermosensitive recording layer with a thickness of 2 to 8 .mu.m which are successively overlaid on the support in this order, as disclosed in Japanese Laid-Open Patent Application 3-130188. According to this application, the image formed on the reversible thermosensitive recording layer can be easily recognized owing to the light reflection layer formed thereunder, and at the same time, the decrease of output from the magnetic recording layer can be prevented by limiting the total thickness of the layers provided over the magnetic recording layer to 10 .mu.m or less.
Further, a magnetic recording medium with a thermosensitive display portion is proposed as disclosed in Japanese Laid-Open Patent Application 5-177975, which comprises a support, a deposited magnetic recording layer with a thickness of 2 to 10 .mu.m provided on the support, and a reversible thermosensitive recording layer with a thickness of about 7 .mu.m directly provided on the magnetic recording layer. The layer structure of this magnetic recording medium is simplified to decrease the thickness of the layers provided over the magnetic recording layer to 10 .mu.m or less. Thus, the magnetic recording performance can be improved without impairing the thermal recording performance.
In any of the above-mentioned recording media, the reversible thermosensitive recording layer is provided on the magnetic recording layer, and the total thickness of the layers provided over the magnetic recording layer is decreased to improve the magnetic recording performance. However, these recording medium have the shortcomings that the image forming performance of the reversible thermosensitive recording layer cannot be enhanced, and the recording output performance from the magnetic recording layer cannot be improved.
To eliminate the above-mentioned shortcomings, there is a demand for a recording medium in which no layer is provided on the recording layers except a thin-layered protective layer. As disclosed in Japanese Laid-Open Patent Application 4-220400, it is proposed to provide a magnetic recording layer in a part of a reversible thermosensitive recording layer. However, the structure of the layers and the method of producing such a recording medium are not specifically described in the application, and the durability of the obtained magnetic recording layer is not clarified.
The recording medium comprising the magnetic recording layer and the reversible thermosensitive recording layer is regarded as useful when applied to the cash card or the like. It is therefore supposed that this kind of recording medium be often allowed to manually pass through a magnetic reader while in use. In light of the usage of this kind of recording medium, the durability of the magnetic recording layer becomes one of the most important factors in the recording medium in which the magnetic recording layer is provided in a part of the reversible thermosensitive recording layer.
Representative examples of the conventional recording medium which comprises a magnetic recording layer and a reversible thermosensitive recording layer which are provided on the same side of a support will now be explained by referring to FIG. 9.
A recording medium Q as shown in FIG. 9, which is disclosed in the previously mentioned Japanese Laid-Open Patent Application 4-220400, comprises a support 1, and a light reflection layer 2, a reversible thermosensitive recording layer 3 and a protective layer 4 which are successively provided on the front side of the support 1, and in addition, a magnetic recording layer 5 is partially provided on the protective layer 4.
The structure of layers provided on the front side of a support 1 in a recording medium S of FIG. 9 is the same as that of the recording medium Q. In the recording medium S, there are provided another magnetic recording layer 9 and another protective layer 10 on the back side of the support 1, opposite to the light reflection layer 2 with respect to the support 1. The recording medium S is a typical example which comprises a thermosensitive recording layer 3 and a magnetic recording layer 9 provided on the opposite sides of the support.
The recording media Q and S as shown in FIG. 9 have the shortcoming that the adhesion between the protective layer 4 and the magnetic recording layer 5 is so weak that the durability of the magnetic recording layer 5 is low.
Further, when the magnetic recording layer is provided on the reversible thermosensitive recording layer, improvements of an adhesive for use in the magnetic recording layer are proposed. However, improvements of the adhesive are limited because the adhesive may seriously affect the reversible thermosensitive recording layer. In addition, there is proposed the improvement of the protective layer 4 in order to increase the adhesion between the magnetic recording layer 5 and the protective layer 4 in the recording medium Q or S as shown in FIG. 9. In this case, however, the repetition durability of the reversible thermosensitive recording layer deteriorates. Such a conventional recording medium Q or S, in which the magnetic recording layer 5 is provided on the protective layer 4 lacks reliability in terms of the repetition durability because the magnetic recording layer 5 is easily peeled from the protective layer 4 or readily takes scratches to impair the functions of the magnetic recording layer 5 when the recording medium is only used several times.
A recording medium R as shown in FIG. 9, which is the same as disclosed in Japanese Laid-Open Patent Application 3-130188, comprises a support 1, and a magnetic recording layer 9, a smoothening layer 11, a light reflection layer 2, a reversible thermosensitive recording layer 3 and a protective layer 4 which are successively overlaid on the support 1. In this case, the protective layer 4, the reversible thermosensitive recording layer 3, the light reflection layer 2 and the smoothening layer 11 hinder the operation for writing information in the magnetic recording layer 9 and reading the information therefrom by use of a magnetic head. To prevent the errors due to the decrease of magnetic output from occurring, it is necessary to decrease the total thickness of the above-mentioned layers 4, 3, 2, and 11 to 5 .mu.m or less.